The p16INK4a gene product acts as a negative regulator of the cell cycle by binding to cyclin-dependent kinases (CDKs) 4 and 6, thereby inhibiting the formation of an active CDK/cyclin D complex. Deletion of the p16 locus has been observed in tumor cell lines and, less frequently, in primary human neoplasms. We analyzed 31 glioblastomas and identified 6 cases with hemizygous and 6 with homozygous deletions of the p16 locus. Eight of these cases showed a concurrent amplification of the EGFR gene (epidermal growth factor receptor) while the overall frequency was 35%. This close correlation suggests that deletion of the p16 chromosomal region constitutes another genetic hallmark of the primary glioblastoma, which rapidly develops de novo, without a less malignant precursor lesion and for which EGFR amplification is a characteristic genetic change. The p16 protein was not detectable in 15 of 22 glioblastomas but only 4 of these showed homozygous deletion of the gene. The alternative transcript p16 beta, for which a growth-suppressing function has been suggested, was co-expressed with p16 alpha mRNA in most cases. Hypermethylation of CpG islands in the 5' region of the p16 gene was identified in only 1 case, suggesting that this alternative mechanism of gene silencing is rarely responsible for loss of p16 expression in glioblastomas. Likewise, only 1 glioblastoma carried a p16 mutation and in addition, unexpectedly, a homozygous deletion of p16 in approximately 80% of tumor cells. This mutation, Arg24Pro, has previously been identified in a melanoma kindred.